Optical polarizing device and laser polarization device

ABSTRACT

The invention concerns a light reflecting element comprising a substrate ( 10 ), a multilayer mirror ( 20 ) and optical coupling means ( 32, 40 ) comprising a diffraction grating ( 40 ); whereby the reflection coefficient of one polarization is damped without damping the reflection coefficient of the orthogonal polarization over a broad wavelength range and with a large tolerance on the optogeometrical parameters of the device.

This application claims benefit of 60/146,806 filed Aug. 2, 1999.

BACKGROUND OF THE INVENTION

This invention relates to multidielectric mirrors and to couplingdevices, in particular for use in a laser device. It also concernsoptical devices comprising a multilayer mirror and a grating, saiddevice being of high polarization selectivity, of particularly largestructural tolerance, and of particularly large wavelength range.

A microchip laser polarization device is known from the market(Nanolase, Grenoble, France) whereby a mechanical stress inducedtransversally on a Nd:YAG microchip laser favours laser emission withthe electric field polarized along the applied external force. Thedisadvantage of such solution is that it is a one by one solutionapplied to an otherwise batch production process of microlasers. Suchsolution is also limited because it can practically only lead to auniform distribution of the electric field.

Another device is known from the scientific literature (V. N.Bel'tyugov, S. G. Protsenko, Y. V. Troitsky, “Polarizing laser mirrorsfor normal light incidence ”, Proc. SPIE, Vol. 1782, 1992, p. 206)comprising a multilayer mirror composed of at least one corrugatedinterface between layers whereby the grating couples the undesiredpolarization of a gas laser into a guided mode of the multilayer, andinduces a differential loss in the laser cavity between the coupledundesired polarization and the uncoupled desired polarization.

The practical limitations of this device are following.

In case one interface only is corrugated, the coupling efficiency into aguided mode of the multilayer is too weak for the device to be appliedto microchip lasers where the beam diameter is 100 μm or less;furthermore, the grating would in such case diffract the lasingpolarization into diffraction orders propagating into the high indexactive crystal substrate. This results in non-acceptable losses.Moreover, the weakness of the coupling efficiency has the firstconsequence that the linewidth of the coupling phenomenon is very narrowand prevents the polarizing effect to be effective over a widewavelength range, as for instance over the full gain bandwidth of aboutone nanometer of Nd:YAG lasers; a second consequence is that thespectral position of the narrow line at which the desired polarisingeffect occurs is highly dependent on the multilayer characteristics,thus on the fluctuations of its manufacturing conditions, and on theenvironmental dependence of the refractive index of the layersespecially on humidity and temperature. This renders the device of thestate of the art unusable in practice since it would require individualpost-trimming and temperature control. In case all, or a large number ofinterfaces are corrugated, the coupling efficiency is increased butthese corrugations lead to a perturbation of the layer depositionconditions which will be even less reproducible and provoke scatteringlosses on the lasing polarization.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful grating device such as a polarizing mirror or apolarizing coupling device in which the above described problems areeliminated.

Another, more specific object of the invention is to provide tolerantcoupling means comprizing a grating capable of damping one polarizationof a laser beam over a large wavelength range while maintaining theresulting scattering at a low level.

A first embodiment of a device according to the invention is an opticaldevice comprising a substrate, a multilayer mirror, a pair of low andhigh refractive index layers, and a corrugation grating in said highrefractive index layer.

Such a device can be used as a laser coupler or as a coupler in a laserdevice.

The combination of the pair of low and high refractive index layer, andof the corrugation grating in said high refractive index layer, resultsin a reduction of the reflection coefficient of a first polarization, bymeans of a destructive interference in the multilayer mirror for thisfirst polarization, with essentially no change of the reflectioncoefficient for the other (second) polarization.

A beam of light is directed toward the optical device according to theinvention through said substrate. In other words, the light beam isincident from the substrate side. The beam then successively traversesthe multilayer mirror and the pair of low and high refractive indexlayers. One polarisation of the beam is reflected as in the absence ofthe grating. The other polarisation is reflected differently due to thegrating, which is placed or made on, or in, the last high index layer,for example at the air side.

The multilayer mirror reflects both polarizations equally. Thepolarizing function is performed by the grating substructure comprizingthe said corrugated pair of low and high refractive index layers.

For a corrugation grating on the last layer to have a non-negligibleeffect on the lasing condition in the cavity, the polarization selectioneffect can not simply be the coupling of one polarization into a guidedmode of the multilayer as disclosed in the state of the art, for examplein the article by Bel'tyugov cited above. The device according to theinvention teaches the use of an abnormal reflection from the last highindex waveguide grating under normal incidence: that is, when theincident polarization is coupled into the last high index layer mode,the optical power is reflected back from the grating with a phaseshiftof π. Thus, the reflection of the damped polarization is not damped byits coupling into a waveguide mode, but it is efficiently reflected backinto the cavity with a π phaseshift by virtue of the said abnormalreflection, thus giving rise to an appreciable degree of destructiveinterference in the multilayer mirror for the coupled polarization, andconsequently inducing on the latter a significantly more efficientdamping.

Those familiar to the art will not be tempted to place the grating onthe side of the last layer of the multidielectric mirror, for example atthe air side, because the field is much weaker there than in the firstlayers, and because, in case of use in a laser device it is believedthat the coupling of the polarization to be damped into a waveguide modenearly outside the laser resonator will hardly have an influence on theintra-cavity polarization lasing conditions.

The substrate can be a laser active material substrate, for example theactive material of a microchip laser.

The first embodiment of the device according to the invention can beadvantageously used at one side of a laser cavity. Its efficiency is sohigh that the necessary damping of the coupled polarization can beobtained with a grating of very small depth, thus leading to a reducedscattering.

The efficiency of the grating is high because the last high index layer,acting as a waveguide, concentrates the modal field in the said layer,and in particular in the grating zone. The high radiation efficiency ofthe grating reduces the quality factor of the wavelength resonance andgives rise to a large wavelength tolerance of the grating mode coupling.

The first embodiment of the invention being substantially lossless, thedamped polarization is not necessarily filtered out. It can thus be alossless polarization filter exhibiting two different reflectioncoefficients for the two incident polarisations.

The polarization device according to the above first embodiment offersthe following specific advantages:

it prevents the diffraction of the uncoupled polarization into thesubstrate, in particular in the case of a microchip laser,

the grating causes little scattering since it is not at the substrateside; in the case of a laser, it is not used at an active medium sidewithin the laser resonator but at the air side,

it can provide a substantial and controllable difference between thereflection coefficient of the two polarizations without inducing anypower loss on the polarization having the smaller reflectioncoefficient.

it can induce the polarization selection of very narrow beams,

it has a particularly large spectral bandwidth.

The second embodiment of the invention concerns an optical mirror,comprising a multilayer mirror, a grating and a metal or metallizedsubstrate or a metal coated substrate, the multilayer mirror beinglocated between the substrate and the grating.

The reduction of the reflection coefficient of the damped polarizationis achieved by means of the coupling of the latter to one of the lossymodes of said multilayer deposited onto said substrate. In a laserdevice, the multilayer is at the substrate side which is at the insideof the laser cavity.

Whereas the coupling of the damped polarization to a guided mode of themultilayer according to the state of the art (V. N. Bel'tyugov et al)leads to a narrow band coupling, the device according to the secondembodiment of the invention leads to a broader band coupling since aguided mode of the air and metal bordered multilayer waveguide sufferslarge losses.

Coupling to a TM mode is preferred. However, coupling to a TE mode isalso possible, although the coupling linewidth is narrower.

The parameters of a structure achieving high and wavelength tolerantabsorption can be found out by means of an available diffractionmodeling code. The parameters can for instance be the number of layersof the multilayer, its first and last layers (low or high index), thetype of metal, the thickness of the metal film, the period and depth ofthe grating.

The specific advantages of the second embodiment of the invention for alaser mirror are the following ones:

the metal borded multilayer offers a larger number of possible TM modes,and new types of modes which do not exist in the coupler of the state ofart: two plasmon modes and all modes of effective index smaller than thesubstrate index ns,

the propagation constants of the TE modes and TM modes are interleaved,thus the lasing TE polarization does not suffer losses,

a metal substrate is compatible with possible fluid cooling in highpower applications,

the coupled mode loss, thus the polarization filtering bandwidth, can beadjusted through the choice of the proper metal,

the metal substrate or film is placed beyond the last layer of themultilayer which usually has a large number of layers. Therefore thefield of the incident lasing beam at this location is close to zero.

The third embodiment of the invention concerns both a mirror and acoupler. Alike in the second embodiment, the multilayer is at thesubtrate side facing the laser cavity, the grating being in the lastlayer. Here, the device comprizes the grating, a multilayer, and thesubstrate, the grating having a period such that the incident dampedpolarization is coupled to one of the first order leaky modes of themultilayer of the same polarization. Leaky modes are transverseelectromagnetic field resonances with total reflection at the air sideand high partial reflection at the substrate side. The leakage of powerin such resonance causes the coupling into such mode to be broadband andtolerant. Since the propagation constants of leaky modes of the twoorthogonal polarizations are interleaved, the structure achieving highand wavelength tolerant leakage for the damped polarization andessentially zero leakage for the lasing polarization can be found out bymeans of codes available on the market.

The structure parameters which can be adjusted are for instance thenumber of layers in the multilayer, and/or the type of its first andlast layers (low or high index), and/or the depth and period of thegrating, and/or the polarisation.

The specific advantages of this third embodiment are the following ones:

the grating period is relatively large, thus easier to fabricate, and isessentially unique and predetermined since the first order leaky mode ofso large a multilayer waveguide has an effective index very close tothat of the substrate,

the third embodiment can be used as both a laser mirror and a lasercoupler,

the flux resistance of the third embodiment is large since the leakymode field does not exhibit a large amplitude anywhere in the structure,and since the power loss mechanism is not of an absorptive nature.

Beyond the specific advantages of the three embodiments which have beenlisted above, the common advantages of all three embodiments of theinvention are the following ones:

the multilayer mirror of the device according to the invention can bethat of a standard laser mirror,

technologically, the grating fabrication step comes after the wholestack of layers have been deposited,

the polarization filtering is performed by a device which can beproduced by batch planar technologies,

the grating device according to the invention can define linearpolarization distributions which are different from rectilinear.

For widening the wavelength range over which the reflection coefficientfor one polarization is decreased, a device according to any embodimentof the present invention is so designed that the grating causes asubstantial fall of the quality factor of the coupled mode for theincident beam of he said polarization.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of he presentinvention will be more apparent from the following detailed descriptionwhen read in conjunction with the accompanying drawings in which:

FIG. 1 is the cross-section view normal to the grating lines of thefirst embodiment of the device according to the invention;

FIG. 2 is the spatial frequency diagram of a particular case of thefirst embodiment of the device according to the invention;

FIG. 3 and 4 are experimental results obtained with a device accordingto the invention,

FIG. 5 is the cross-section view normal to the grating lines of thesecond embodiment of the device according to the invention;

FIG. 6 is the cross-section view normal to the grating lines of thethird embodiment of the device according to the invention;

FIG. 7 is the spatial frequency diagram of the case of the thirdembodiment of the device according to the invention;

FIGS. 8a) and b) is the plane view of a grating device according to theinvention providing a radial and an azimuthal distribution of thepolarization;

FIG. 9 is the cross-section view of a grating device according to theinvention providing a polarization selection at two emissionwavelengths;

FIG. 10 is the plane view of a grating device according to the inventionproviding a polarization selection at more than two wavelengths;

FIG. 11 is the plane view of a device according to the inventionproviding cross-polarized emission at two wavelengths;

FIG. 12 is the plane view of another device providing cross-polarizedemission at two wavelengths;

FIG. 13 is the plane view of another device providing cross-polarizedemission at two wavelengths;

FIG. 14 is the cross-section view of a device according to the inventionfor gas lasers;

FIG. 15 is the cross-section view of a device according to the inventionon a curved substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several detailed embodiments of the invention are given herebelow. Eachembodiment comprises a multilayer structure. A “mode” of such amultilayer structure is a transverse electromagnetic field resonancepropagating in the direction of the planes of the layered structure. Thespectral linewidth of this spatial resonance is related with thestrength of its coupling with the incident beam, and/or with its ownabsorption or leakage losses.

FIG. 1 is the cross-section view of a first embodiment of the deviceaccording to the invention. An incident beam IB of spectral width B ispreferably directed from the substrate side.

The device according to this first embodiment is composed of a substrate10, a multilayer mirror 20, a pair of layers 30, and a diffractiongrating 40.

The substrate 10, of upper surface 11, can be an amorphous material likeglass. It can also be a laser crystal like YAG or a birefringentcrystal.

The multilayer mirror 20, which can be a standard one, comprizes thenumber of layers of alternate low and high refractive index needed toprovide the desired reflection coefficient over the desired bandwidth.

The last layer 21 of the multilayer mirror 20 can be of high index n_(h)or low index n₁. Its thickness is t_(s); layer 21 is often a λ/4 layerwhich means that t_(s)=λ/(4n_(h)) or t_(s)=λ/(4n₁) essentially.

An additional optical coupling means comprizes the pair of layers 30deposited by standard dielectric film techniques like ion plating,electron beam evaporation, sputtering, or ion assisted sputtering. Thispair of layers 30 comprizes a first layer 31 of refractive index n₁ (lowrefractive index) and thickness t₁, and a second layer 32 of index n_(h)(high refractive index: n_(h)>n₁) and thickness T_(h).

The diffraction grating 40 is etched into the second layer 32. Grating40 comprizes grooves 41 of period Λ and depth d. The second layer 32 isan optical waveguide of index n_(h). It propagates at least the TE₀ modeat the central wavelength λ₀ of the laser mirror 20. It can alsopropagate other modes, in particular when its thickness is increased. Apolarisation can be damped or filtered by the device if it couples toone mode propagated by the second layer or the optical waveguide 32.

In order to perturb the operation of the multilayer mirror 20 as less aspossible, the thickness and index of the additional pair of low and highindex layers 31, 32 are, regardless of the effect of the grating 40,preferably set to an additional optical path providing a total roundtrip additional optical phase equal to an integer number of essentially2π at the wavelength of operation λ (within spectral bandwidth B):$\begin{matrix}{{{2\quad \frac{2\quad \pi}{\lambda}\quad ( {{t_{h}\quad n_{h}} + {t_{l}\quad n_{l}}} )} = {2m\quad \pi}},{m = 1},{{or}\quad 2},{{or}\quad 3}} & (1)\end{matrix}$

where t_(h) is the average thickness of the last high index layer 32(seen by an optical wave incident normally onto the substrate surface11), t₁ is the thickness of the last low index layer 31, and n_(h), n₁,are the refractive index of the last high and low index layers 32 and 31respectively.

Equation (1) holds for the case where the last layer 21 of themultilayer is of high index. When this last layer 21 of the multilayeris of low index, the following expression should be used:$\begin{matrix}{{{2\quad \frac{2\quad \pi}{\lambda}\quad ( {{t_{h}\quad n_{h}} + {t_{l}\quad n_{l}}} )} = {( {{2m}\quad - 1} )\quad \pi}},{m = 2},{{or}\quad 3},} & ( 1^{\prime} )\end{matrix}$

In the presence of the polarizing grating of depth d, t_(h) is definedas the average thickness of the last high index layer 32 at the linelocated in the middle of the grating corrugation (see FIG. 1) in casethe line/space ratio of the grating corrugation is 1:1 (in this case, itis essentially equal to t_(h)=T_(h)−d/2).

If the corrugation line/space ratio is not 1:1, t_(h) is defined at aline located between the bottom and the top of the grooves depending onthe duty cycle of the grating grooves.

In a preferred embodiment, the index n_(h) and the thickness t_(h) ofthe last high index layer are large enough so that the effective indexn_(e) of the mode of the last high index layer 32 to which the undesiredpolarization is coupled is larger than the substrate index n_(s);otherwise the reflected (undamped) polarization could be diffracted inthe higher index substrate 10.

The materials of the last two layers 31 and 32 are advantageously thesame as the materials of the multilayer mirror 20. But they can bedifferent if needed, for instance if the refractive index of the higherindex material used in the multilayer mirror 20 is too low.

If m=2, the above condition (1) implies that the thickness t_(h) and t₁of the last two layers 31, 32 (which thickness increases with m) islarger or significantly larger, by a factor of about 2, than thethickness of the λ/4 layers of a standard multilayer mirror 20. Thismeans in particular that the fundamental mode of the last high indexlayer 32 has a large effective index n_(e) which increases with anincreasing thickness of layer 32. Therefore, its modal field isessentially confined in the last layer and hardly sees the high indexlayers of the multidielectric mirror; besides, if n_(h) is larger orsignificantly larger than the substrate index n_(s), the effective indexn_(e) of the coupled mode, which effective index depends on both n_(h)and the layer thickness, will more surely be larger than n_(s).

If m=1, the high index layer optical thickness 2πn_(h)t_(h)/λ ispreferably strictly larger or significantly strictly larger than λ/4layers of the standard multilayer mirror 20, the optical thickness2πn₁t₁/λ of the low index layer 31 being accordingly smaller than thethickness of the λ/4 layers of the standard multilayer mirror 20, toallow the effective index n_(e) of the fundamental mode of the dampedpolarization to be larger than the refractive index n_(s) of thesubstrate, and to allow a confinement of the coupled mode essentially inthe last high index layer 32.

FIG. 2 is the spatial frequency diagram of the first embodiment of theinvention or of the coupling means of this first embodiment. In thespatial frequency domain, the first grating order coupling condition(synchronism condition) between the normally incident free space waveand a guided mode, for abnormal reflection, writes

K _(g) =n _(e) k ₀(2),

where K_(g)=2π/Λ is the grating spatial frequency, k₀=2π/λ is thespatial frequency of a free space wave in vacuum (λ being one wavelengthwithin spectral bandwidth B), and n_(e) is the so-called effective indexof the coupled mode of the damped polarization.

It is apparent from FIG. 2 that the grating coupling to any mode of agiven polarization of layer 32 which has n_(e)<n_(s) causes diffractionof the lasing orthogonal polarization into the substrate 10, thuscausing undesired losses on the lasing polarization. Any waveguide modewith n_(e)>n_(s) can be the mode chosen for filtering out the dampedpolarization where n_(s) stands for the refractive index of an isotropicsubstrate or for the corresponding refractive index of a birefringentsubstrate. This explains the condition set above on n_(e) and n_(s).

For abnormal reflection to take place, the product of the fieldradiation coefficient α of the waveguide grating by the incident beamdiameter w, αw, is preferably larger than unity:

αw>1  (3)

For example: αw=2π.

For narrow beam diameters, this sets a requirement for an unusuallylarge radiation strength of the waveguide grating. For the abnormalreflection to efficiently operate on a small diameter beam as that of amicrochip laser for example, the radiation strength, described by thewaveguide grating radiation coefficient α, is thus preferably strong.

Another condition is known for a preferably high abnormal reflection totake place: it states that the absorption and scattering losses of thewaveguide layer are smaller or significantly smaller than the radiationloss of the grating.

More generally, the thickness t_(h) of the last layer as well aspossibly its index n_(h) are adjusted so as to give rise to the largestpossible radiation coefficient α. This can be made by maximizing thelast layer waveguide modal field in the grating region, and in realizinga constructive interference condition between the outwards and inwardsdiffraction product taking place in the grating waveguide. This leads toa condition on t_(h), and possibly on n_(h), and in turn dictates theratio between t_(h) and t₁ if the total phaseshift between the top ofthe standard multilayer mirror and the middle line of the gratingcorrugation is maintained close to 2 mπ (see equation (1) above).

For instance, the condition of maximum α in case the damped polarizationis the TE polarization (the electric field is parallel to the gratinglines) mainly amounts to optimizing the effective index dependent term(n_(h) ²−n_(e) ²)/(n_(e)t_(eff)) of the known analytical expressiongiving the radiation coefficient α of a TE mode upon radiation in thedirection normal to the waveguide by a sinusoidal grating of depth 2s interms of the opto-geometrical parameters of the waveguide:

α=(k ₀ s/2)²(n _(h) ² −n _(e) ²)(n _(h) ²−1)(n _(h) ²−(n _(h) ² −n ₁ ²)sin²(n _(h) k ₀ t _(h)))/(n _(e) t _(eff)(n _(h) ²(n ₁+1)²+(n _(h)²−1)(n _(h) ² −n ₁ ²) sin²(n _(h) k ₀ t _(h)))),  (4)

In this expression, k₀, n_(h), n₁, n_(e), t_(h) have the same meaning asalready indicated above. t_(eff) is the effective thickness of knowneffective thickness of a TE mode given by

t _(eff) =t _(h)+1/(k ₀(n _(e) ² −n ₁ ²)^(1/2))+1/(k ₀(n _(e)²−1)^(1/2)).

The analytical formulae giving the field radiation coefficient α of asinusoidal grating made in a step index waveguide for TE and TM guidedmodes can be found in the article by V. A. Sychugov et al. “Lightemission from a corrugated dielectric waveguide”, Sov. J. QuantumElectron., Vol.10, N° 2, Feb. 1980, p.186-189.

In the case of the TE polarization, α takes the form (4) given above.

In the case it is preferable to damp the TM polarisation instead of theTE polarisation, the radiation coefficient α is also given in the abovementioned article.

These expressions can be used for non-sinusoidal profiles as well: in anon-sinusoidal grating profile, s is the amplitude of the firstharmonics in the Fourier series of the grating profile. For instance, inthe case of a rectangular grating profile of depth d, of line/spaceratio 1:1, s=0.5(4d/π), or the depth d of a rectangular grating givingthe same α as a sinusoidal profile is simply d=(2s)π/4.

With the example of low and high index layers of SiO₂ and Ta₂O₅ (havingan index of, respectively, 1.48 and 2.18 at 1.06 μm wavelength) made byion plating, a typical radiation coefficient a of several hundreds cm⁻¹can be obtained with a 130 nm deep grating. This sets abnormalreflection conditions for a beam as narrow as w=100 μm which correspondsto the typical beam width of microchip lasers.

The additional pair of layers 31, 32 with the grating 40 on the highindex layer 32 form a coupling means which causes a major damping of thecoupled polarization. First, it reflects the incident wave with aphaseshift of π, characteristics of abnormal reflexion, for the coupledpolarization; secondly, the diffraction efficiency giving rise to thesaid out-of-phase reflection can be optimized by optimizing the highindex layer thickness under the condition of 2 mπ total round tripphaseshift through the pair of layers. This ensures that a gratingplaced at the side of the multilayer mirror opposite to the substratecan provoke a sufficient damping of the coupled polarization withoutaffecting the other polarization.

The initial thickness T_(h) of layer 32, then the grating thickness d,are preferably determined so as to provide the maximum radiationcoefficient α with the shallowest grating depth d satisfying thecondition αw>1 for efficient resonant reflection for a normally incidentfree space wave of prescribed diameter w.

From the determined average thickness t_(h) of the layer 32, thethickness t₁ of the low index layer 31 is determined from:

t ₁=(0.5λm−n _(h) t _(h))/n ₁  (5),

which equation results from equation (1) above.

However, if the last layer 21 of the standard multilayer is of lowindex, layer 21 is considered as a part of the low index layer 31; inthis case, the contribution of layer 21 to the total round tripphaseshift of the layer pair 30 is essentially π since layer 21 isessentially a λ/4 layer; therefore, in case layer 21 of the standardmultilayer is of low index, the condition on t_(h), n_(h), t₁, n₁ forthe preservation of the reflection characteristics of the standardmultilayer 20 is:

n ₁ t ₁=(2m−1)λ/4−n _(h) t _(h) , m=2, or 3,  (6),

which equation results from equation (1′) above.

Let n_(e) the effective index of the dominant mode of the undesiredpolarization of the waveguide layer 32 with grating 40. The period Λ ofgrating 40 essentially satisfies the synchronism condition Λ=λ/n_(e) forthe coupling of a free space wave at wavelength λ (one wavelength withinspectral bandwidth B) under normal incidence to the waveguide mode ofeffective index n_(e). This coupling is wavelength tolerant since therequest for a large α for efficient anomalous reflection gives rise to awide resonance in the waveguide mode excitation according to theapproximate expression Δλ=αΛλ/π for the spectral width Δλ of theanomalous reflection. This illustrates one of the advantages of thedevice according to the invention whereby this characteristics ofwavelength tolerance permits its manufacturing without individualpost-trimming.

The grating 40 can also be realized by photoinscription in the highindex layer 32. In this case, t_(h)=T_(h)/2, and the waveguide thicknessT_(h) giving the largest radiation coefficient α is the known waveguidethickness of maximum field confinement from where t_(h), n_(e), then t₁,and finally Λ can be derived.

The design procedure of a device according to the invention can be thefollowing one.

A refractive index n_(h) of layer 32 is chosen, preferably the same asthe high index material involved in the multilayer mirror 20, but largeror significantly larger than n_(s).

Then, the thickness of layer 32, and possibly its index n_(h), as wellas the grating depth d, are chosen so as to achieve a radiationcoefficient α of at least 1/w of a grating placed at the interfacebetween layer 32 and air. If d/Λ<15%, known analytical formulae based onthe Rayleigh approximation can be used to calculate a as that givenabove for the TE modes. If d/Λ>15%, the designer will resort toavailable codes solving the diffraction problem exactly, like GSOLVER©from Grating Solver Development Company, Allen, Tex. 75002.

The thickness n₁ of layer 31 can then be deduced from the condition ofessentially 2 mπ additional round trip phaseshift for a wave penetratingthe pair of layers 30 from the multilayer mirror and reflected at thewaveguide grating 40 when layer 21 is of high index (m=1, 2, 3), andessentially (2m−1)π., m=2, 3 . . . when layer 21 is of low index.

As an example of the device according to the invention, the polarizinggrating of a Nd:YAG microchip laser emitting at λ=1.064 μm comprises amultilayer mirror 20 of 95% reflection using 11 layers of Ta₂O₅ (0.1217μm thickness and 2.186 index) and SiO₂ (0.1795 μm thickness and 1.482index), the first layer and the last layer being made of Ta₂O₅,deposited by ion plating, and a pair 30 of additional layers of SiO₂ andTa₂O₅ with t₁=0.4 μm and t_(h)=0.25 μm, and a rectangular profile ofperiod Λ=0.582 μm and depth d=0.13 μm (T_(h) is 0.31 μm). The radiationcoefficient α of the TE₀ mode is several tens of μm⁻¹ which is adequateto give rise to efficient abnormal reflection for beam diameters wlarger than about 100 μm. It can be checked that the total phaseshiftfor a round trip within the pair of layers 30 is about 1.12 (4π). Asanother example, where the total phaseshift for a round trip in the pairof layers 30 is only essentially 2π, instead of 4π, and the last highindex layer 32 is the same as in the first example, the last low indexlayer 31 giving the maximum αhas t₁=0.04 μm.

A device according to the first embodiment damps a polarisation asillustrated on FIG. 1. Incident beam IB comprises two polarisations P1and P2. P1 is reflected by the multilayer mirror 20 without beingcoupled by grating 40 into any mode of high index layer 32. When thedevice is a mirror of a laser device, P1 is thus the lasing beam 43inside the laser cavity. P2 is also reflected by the multilayer mirror20, but is coupled to waveguide or high index layer 32 by grating 40. Ascan be seen on FIG. 1, that part of beam P2 which is thus coupledreturns back into the cavity with a phase shift of π. This explains whydestructive interference in the multilayer damps this polarisation.

One example of a device according to the first embodiment wasinvestigated in an open cavity scheme. A 1″ Nd:YAG wafer of 500 mmthickness was coated at one side with a SiO₂/Ta₂O₅ multilayer systemdeposited by ion plating. The resulting refractive index at 1064 nmwavelength is 1.48 and 2.18 respectively. The multilayer comprises a setof 11 alternate λ/4 layers, starting with a SiO₂ layer at the waferside. This standard multilayer was designed for a reflection coefficientof 94% at 1064 nm. A pair of SiO₂/Ta₂O₅ layers of essentially π totaloptical thickness was added at the air side as the polarising pair. Thisdecreases the reflection coefficient to 90%.

The high index layers of the multilayer can be considered as awaveguide. The dominant TE₀ mode is however mainly concentrated in thethicker last high index layer. The effective index n_(e) of the dominantTE₀ mode of the multilayer mirror was measured at 1064 nm by mean of theprism coupling technique with a Nd:YAG laser. The period Λ of thecorrugation grating aimed at coupling the emitted TE polarisation to theTE₀ mode was deduced from Λ=λ/n_(e) corresponding to waveguide couplingunder normal incidence. The grating depth into the last Ta₂O₅ layerneeded to achieve a high radiation coefficient was determined by meansof a multilayer grating code based on a generalised source method asexplained in the article of A. V. Tishchenko, ‘A generalised sourcemethod for wave propagation’, Pure Appl. Opt., Vol. 7, 1998, pp.1425-1449 or in the article by the same author ‘A generalized sourcemethod: new possibilities for waveguide and grating problems’, inOptical and Quantum Electronics, vol.32, pp. 971-980, 2000.

The grating of 1 mm² area was obtained by direct writing in a e-beamresist by means of a LION LV1 lithography system followed by ReactiveIon Beam Etching at a depth of 140 nm. This raises the reflectioncoefficient of the desired TM polarisation to 95%. The choice of thisnominal reflection coefficient is by no means the result of an optimisedlaser design. It was just taken as a benchmark. CW laser would usuallyrequire a significantly larger reflection coefficient whereas pulsedlasers would operate with a lower reflection.

The corrugated part of the Nd:YAG microlaser mirror was tested in acharacterisation experiment comprising the stabilised and collimatedwhite light source of a spectrometer. The beam falls normally onto theYAG wafer through a 1 mm² square hole made in an opaque sheet. A beamsplitter redirects the reflected beam to the entrance slit of amonochromator equipped with a 600 l/mm grating. The signal was detectedby a germanium photo-detector operating at 77° K in combination with acomputer controlled counting system. The incident polarisation can beset to TE (electric field parallel to the grating lines) or to TM bymeans of a polariser placed before the YAG wafer.

FIG. 3 presents the experimental results for the reflection of thepolarising mirror under normal incidence versus wavelength in arbitraryunits of optical power. The reflection difference maximum is found veryclose to 1.064 mm which is the wavelength of a Nd:YAG microchip laser.The dip in the TE curve is the evidence of the desired polarisingeffect. The TM curve is continuous over the whole wavelength range. Thisconfirms that the grating k-vector does not couple the incident TMpolarisation to any TM guided mode of the multilayer mirror, and islarge enough to prevent any propagating TM diffraction order in the airand in the YAG substrate of index 1.82. The TE dip is rather wide as aneffect of the strength of the radiation coefficient α, showing that thepolarisation filtering effect is possibly wide band and can for instancebe used to polarise all the modes of a multi-longitudinal modes higherpower microchip laser having a longer cavity. The depth of thereflection dip is of the order of 50% of the TE amplitude. This is morethan necessary in the present case and can still be optimised. The depthneeded to efficiently prevent the lasing of the TE polarisation dependson the mirror reflection coefficient:the larger the reflectioncoefficient, the shallower the TE reflection dip has to be. Quantitativerecommendations concerning the needed depth of the dip can however notbe made at this preliminary stage since the effect of scattering wasvery low and could not be estimated with sufficient accuracy.

In order to assess the polarising function of the grating mirror as alaser functional element, the microchip cavity was then closed at thepump side. The same wafer Nd:YAG was butted against a glass wafer coatedon one side with a multilayer mirror exhibiting 99% reflection at 1064nm and 94% transmission at the pump wavelength of 807 nm. The other sidehas no anti-reflection coating. Good optical contact between the YAGwafer and the mirror plate was obtained by means of an index matchingthin film of Glycerol. The optical pumping is provided by a multimodepigtailed semiconductor laser delivering a pump power of up to 5 Watt. Alens focuses a fibre output into the microchip laser assembly. Noparticular care was taken to optimise the excitation conditions.

FIG. 4 shows the spectrum emitted by the pumped assembly versuswavelength for two values of the pump power. The dashed curvecorresponds to a super-radiant emission. The ratio between TE and TMemission is close to 1. The solid line corresponds to a pump level abovethreshold. Two peaks can be distinguished within the gain curve. Theratio between the TE and TM polarisations is below −17 dB. This clearlydemonstrates that a grating can be the integrated element whichstabilises the polarisation state of a solid state microchip laser. Noprecise scattering measurement could be performed at that stage.However, systematic measurements of the type of FIG. 4 tend to indicatethat the scattering level on the desired TM polarisation is much below1%. This is expectable since the diffractive structure is not right inthe laser cavity where the field is maximum. Furthermore, it is likelythat an optimised structure will require a grating depth shallower thanthe existing 140 nm, giving therefore rise to an even lower scattering.

It is thus shown that, according to the invention, a grating associatedwith the multilayer mirror of a laser can be made to dictate thepolarisation of the emission. This allows the manufacturing ofpolarisation stabilised microchip lasers to be fully achieved by lowcost planar technologies. This technique can be applied to pulsed aswell as to CW lasers.

The first embodiment of the invention can thus be used at one side of alaser cavity. A laser device further comprises pumping means, like asemiconductor diode for a microchip laser, or an array of semiconductorlasers in solid state lasers.

The invention applies as well to the polarisation control of gas lasers.A device according to the invention, in the form of a singlereflection/polarising element as disclosed above, can be used in a gaslaser in replacement of the intra-cavity Brewster element.

FIG. 5 is the cross-section view of a second embodiment of opticalcoupling means according to the invention.

The incidence of the beam IB of spectral width B, is only from the airside.

The device is composed of a substrate 10, a multilayer 20, a grating 40in the last layer 21 of the dielectric multilayer 20.

The substrate can be composed of a metal of complex relativepermittivity ε_(m); the metal substrate possibly contains cooling means12.

Alternatively, the substrate can be made of a solid material coated witha metal film 33 of thickness t_(m) and complex permittivity ε_(m).Thedielectric mutilayer 20, bordered by air at one side and by the metal 11at the other side, represents a waveguide which can propagate guidedmodes of both TE and TM polarizations along a direction parallel to theplane 11.

These modes are transverse resonances of the electromagnetic field. Forthe same multilayer 20, their number is more than the double of thenumber of the modes propagating in a structure of the state of the artas disclosed in the article of Bel'tyugov cited above.

These modes also have an effective index n_(e) which in the presentembodiment can have values between 1 and close to n_(h) (instead ofbetween n_(s) and close to n_(h) in a structure of the state of theart).

The effective index of the TE and of the TM modes are interleaved.Furthermore, they experience absorption losses since the modal field“sees” the metal boundary. The TM polarised modes experience asignificantly larger attenuation than the TE modes. There are also twoTM modes, called surface plasmon modes, which do not have their TEcounterparts; these are lossy collective electron oscillationpropagating along the metal 11 and along the lower 13 of the metallicfilm.

As a result of the metal absorption, the linewidth Δλ of the moderesonance experiences a broadening. This means that the polarizingfunction can be executed with large tolerances over a rather broadwavelength range.

One can characterize the metal absorption effect on the coupled TM modeby the field absorption coefficient α_(a) of the said TM guided mode,and the radiation coefficient of the chosen TM mode in presence of thegrating by the field radiation coefficient α. The best conditions for atransfer of energy from the incident TM incident polarization to thechosen TM guided mode correspond to α_(a) being essentially equal to α.

In such preferred situation, the wavelength linewidth Δλ expresses asΔλ=αλΛ/π. In a preferred embodiment, the polarization of the incidentbeam IB to be filtered out is the TM polarization with the electricfield directed orthogonally to the grating grooves 41. The period Λ ofgrating 40 is adjusted so as to satisfy the coupling synchronismcondition Λ=λ₀/n_(e) where λ₀ is the central wavelength of the linewidth(within spectral bandwidth B) of the selected propagating TM mode andn_(e) is the effective index of the said selected TM mode at wavelengthλ₀. The selected TM mode is chosen for example on the basis of thefollowing criteria: first, its attenuation due to the metallic border islarge; secondly, there should be no TE near neighbour mode, i.e., λ₀ isnotably outside the linewidth of any neighbouring TE mode. Suchconditions are easy to satisfy by using one of the commerciallyavailable grating computer codes, for example the GSOLVER softwarealready cited above. The same code will give the depth of grating 40leading to the requested decrease of the reflection coefficient of thedamped polarization.

As an example of the second embodiment applied to the case of a lasermirror, the substrate 10 is made of aluminum of complex permittivityε_(m)=(−235, 42.5) at λ₀=1064 nm. The multilayer comprizes 23 layersidentical to those of the first embodiment of the invention. The grating40 of period Λ=0.768 nm and depth 100 nm realized in the last high indexlayer 21 of initial thickness 127 nm couples the TM polarization of thenormally incident beam IB to the TM mode of effective indexn_(e)=1.38542. The nearest TE modes have effective indices of 1.33384and 1.44703. The resulting reflection coefficients are 99.8% for the TEpolarization and 6.1% for the TM polarization at λ₀=1064 nm. Thelinewidth over which the TM reflection coefficient is smaller than 90%is 3.7 nm, i.e., larger than the gain bandwidth in Nd:YAG. This alsomeans for instance that the tolerance of the low layer index is 0.005and that on the high layer index is 0.01.

These tolerances on the low and high index are within reach for mostdielectric deposition technologies.

A device according to the second embodiment of the invention can be usedas one of the mirrors of a laser cavity, the other mirror being astandard multilayer mirror. Such a laser cavity thus comprises an activemedium, and two ends mirrors, one of which having the structuredisclosed in relation to FIG. 5. A laser device further comprisespumping means, like a semiconductor diode for a microchip laser, or anarray of semiconductor lasers for solid state lasers.

FIG. 6 is the cross-section of a third embodiment of a device or ofoptical coupling means according to the invention.

The incidence of the beam IB (of spectral bandwidth B) is from the airside.

The device according to this embodiment is composed of a dielectricsubstrate 10, a multilayer 20, a grating 40 in the last layer 21 of themultilayer 20.

The grating period Λ is adjusted so as to achieve the coupling of one ofthe polarizations of the incident beam IB to one of the first orderleaky modes of the same polarization of the multilayer 20 and thesubstrate 10.

In this embodiment the substrate10 represents an optical power sink forthe leaky mode excited by grating 40 of period Λ=λ₀/n_(e) where n_(e) isthe effective index of the chosen leaky mode. λ₀ is one wavelengthwithin bandwidth B.

The leaky TM and TE modes also have an effective index n_(e) which canhave values between 1 and n_(s) instead of between n_(s) and n_(h) in astructure of the state of the art. The effective index of the TE and TMleaky modes are also interleaved.

The power leakage occurs in the form of two symmetrical beams LMpropagating through the interface 11 between the multilayer 20 and thesubstrate 10 under a grazing angle. The linewidth Δλ of a leaky mode issignificantly broader than that of a true guided mode because thestanding wave in the multilayer is accompanied by a power leak,associated with partial reflection at the interface 11.

Therefore this embodiment is also broadband and is tolerant on themanufacturing conditions of the multilayer 20. One can characterize theleakage effect on the coupled TM or TE mode by the field leakagecoefficient α₁ of the said TM or TE guided mode and the radiationcoefficient of the chosen TM or TE mode in presence of the grating bythe field radiation coefficient α.

The best conditions for a transfer of energy from the incident TM or TEpolarization to the chosen TM or TE leaky mode correspond to α₁ beingessentially equal to α.

In such preferred situation, the wavelength linewidth Δλ expresses asΔλ=αλ₀Λ/π.

Given the standard multilayer of a laser mirror or of a coupler, acalculation by means of an available code identifies which is thepolarization of the first dominant leaky mode, leading to the leastleakage of the second dominant mode of the orthogonal polarization. Onecan refer for example to R. Ulrich et al. Appl. Phys., Vol.1, No.55,1973.

The grating period and depth can be adapted to more efficiently couplethe corresponding incident polarization of beam IB to the said firstdominant leaky mode.

As an example of the third embodiment applied to the case of a lasermirror, the substrate is made of quartz, n_(s)=1.45. The multilayer isidentical to that of the example of the second embodiment of thecoupling means. The grating period Λ=747 nm and depth 70 nm realized inthe last high index layer 21 of initial thickness 127 nm couples the TEpolarization to the TE dominant leaky mode of effective indexn_(e)=1.42436.

The effective index of the dominant mode of the orthogonal polarizationis 1.40129. The resulting reflection coefficients are 73.2% and 99.1%for the TE and TM polarization respectively. The linewidth over whichthe TE reflection coefficient is smaller than 90% is 7.6 nm, i.e. largerthan the gain bandwidth in Nd:YAG.

This corresponds to a tolerance of 0.01 on the low and high multilayerrefractive index. These tolerances are also within reach for mostdielectric multilayer deposition technologies.

A device according to the third embodiment of the invention can be usedas one of the mirrors of a laser cavity, the other mirror being astandard multilayer mirror. Such a laser cavity thus comprises an activemedium, and two end mirrors, one of which having the structure disclosedin relation to FIG. 6. A laser device further comprises pumping means,like a semiconductor diode for a microchip laser.

FIG. 7 is the spatial frequency diagram of the third embodiment of theinvention. The grating 40 of grating constant K_(g)=2π/Λ couples onepolarization of the normally incident beam IB to a leaky mode of themultilayer 20 and substrate 10 represented by a spatial frequency vectork_(LM) making a grazing angle θ with the interface 11.

FIG. 8 is the plane view of a particular arrangement of the grooves 41of grating 40 according to any of the three embodiments disclosed above.

Grating lines 41 of grating 40 are azimuthally distributed.

In FIG. 8a) grating 40 induces a damping of the reflection for a locallyazimuthal polarization of the electric field by coupling it to the TE₀mode of grating waveguide 32 according to the first embodiment of thecoupling means, or to a lossy TE mode of the multilayer 20 according tothe second embodiment, or to a TE leaky mode according to the thirdembodiment.

The polarization distribution of the electric field which experiences noreflection damping is the radial polarization distribution 60.

When placed in a laser resonator, this mirror gives rise to an emittedbeam which has a radial polarization distribution of the electric field.

In FIG. 8b) grating 40 of same pattern as in FIG. 8a), but of differentperiod, induces a damping of the reflection for a locally radialpolarization of the electric field by coupling it to the TM₀ mode ofgrating waveguide 32 according to the first embodiment of the couplingmeans, or to a TM mode of the multilayer 20 according to the secondembodiment, or to a TM leaky mode according to the third embodiment. Thepolarization distribution of the electric field which experiences noreflection damping is the azimuthal distribution 61.

In the first embodiment of the coupling means the effective index n_(e)of the TM₀ mode is made larger than the substrate index n_(s) bysuitably choosing the index and thickness of layer 32. Otherpolarization distributions of the light emitted by a laser can beobtained by suitably distributing the grating lines as long as thepolarization distribution essentially corresponds to a transverse modeof the laser resonator.

FIG. 9 is the cross-section view of a device according to any of theembodiments of the invention, accomplishing the polarizing function at,and in the neighborhood, of two wavelengths λ₁ and λ₂.

The spatial frequency spectrum of grating 40 contains two spatialfrequencies, K_(g1), and K_(g2), each corresponding to the coupling ofone different incident wavelength into the same mode of the opticalcoupling means with two different effective index n_(e1) and ne_(e2) atthe two different wavelengths.

This can be realized by digitizing in a binary format the product of asinusoidal function of high spatial frequency (K_(g1), +K_(g2))/2 by acosine function of low spatial frequency (K_(g1)−K_(g2))/2, K_(g1)satisfying K_(g1) =2πn _(e1)/λ₁, K_(g2) satisfying K_(g2)=2πn_(e2)/λ₂.In the particular case of the first embodiment of the coupling means, λ₁and λ₂ are preferentially not too far apart so that the 2 mπ conditionon the total phaseshift in the pair of layers 30 is still approximatelysatisfied for both wavelengths.

FIG. 10 is the plane view of the device according to any of theembodiments of the invention whereby contiguous spatially multiplexedgrating areas 42 of different spatial frequency induce a polarizationdamping at different wavelengths which match with the emission spectrumof a gas laser for instance. There is no need that one singlepolarization selective grating area covers the whole incident wavecross-section. It suffices that there is sufficient polarization dampingfor the damped polarization of the concerned laser resonator mode. Thereis the need for contiguous grating areas in order to avoid phase frontjumps in the wave front of the emitted waves. Again, in the firstembodiment of the coupling means, the different wavelengths arepreferably not too far apart so that the 2 mπ total phaseshift conditionthrough layer 32 is essentially fulfilled for all wavelengths.

FIG. 11 is the plane view of the device according to any embodiment ofthe invention comprizing two grating areas 43 and 44 of parallelorientation and different periods Λ₁ and Λ₂ respectively. Grating area43 with grating period Λ₁ couples the normally incident free space waveof electric field polarization parallel to the grating lines atwavelength λ₁ to a TE of the coupling means having effective indexn_(e1) at wavelength λ₁ satisfying Λ₁=λ₁/n_(e1). Grating area 43 withgrating period Λ₂ couples the normally incident free space wave ofelectric field polarization perpendicular to the grating lines atwavelength λ₂ to a TM mode of effective index n_(e2) at wavelength λ₂satisfying Λ₂=λ₂/n_(e2). In the first embodiment of the coupling means,the index and thickness of layer 32 are preferably such thatn_(e2)>n_(s). A laser having two emission lines at λ₁ and λ₂, or havinga gain bandwidth containing λ₁ and λ₂, emits λ₁ with a polarization ofthe electric field perpendicular to the grating lines, and λ₂ with thepolarization of the electric field parallel to the grating lines.

FIG. 12 is the plane view of a device according to any of theembodiments of the invention comprizing two grating areas 45 and 46 ofperpendicular line orientation and different periods Λ₁ and Λ₂. Gratingarea 45 with grating period Λ₁ couples the normally incident free spacewave at wavelength Λ₁ of electric field polarization parallel to thegrooves of grating area 45 to a TE mode of the coupling means ofeffective index n_(e1) satisfying Λ₁=λ₁/n_(e1). Grating area 46 withgrating period Λ₂ couples the normally incident free space wave atwavelength λ₂ of electric field polarization parallel to the grooves ofgrating area 46 to a TE mode of effective index n_(e2) satisfyingΛ₂=λ₂/n_(e2). A laser having two emission lines at λ₁ and λ₂, or havinga gain bandwidth containing λ₁ and λ₂, emits λ₁ with a polarization ofthe electric field perpendicular to the grating lines of grating area45, and λ₂ with the polarization of the electric field perpendicular tothe grating lines of grating area 46.

FIG. 13 is the plane view of another device according to the threeembodiments of the coupling means comprizing two orthogonally crossedgratings 47 and 48 of periods Λ₁ and Λ₂. Grating 47 couples thepolarization of the normally incident free space wave of wavelength λ₁parallel to the lines of grating 47 to a TE mode of the coupling meansof effective index n_(e1) at wavelength λ₁. Λ₁ satisfies the matchingcondition Λ₁=λ₁/n_(e1). Grating 48 couples the polarization of thenormally incident free space wave of wavelength λ₂ parallel to the linesof the grating 48 to a TE mode of effective index n_(e2) at wavelengthλ₂. Λ₂ satisfies the matching condition Λ₂=λ₂/n_(e2). To prevent anydiffraction loss of the incident free space wave into the substrate 10in the case of the first embodiment, the effective index n_(e1) atwavelength λ₁ and n_(e2) at wavelength λ₂ preferably satisfy therelationship ((n_(e1)/λ₁)²+(n_(e2)/λ₂)²)^(1/2)<n_(s)/min (λ₁, λ₂).

FIG. 14 is the cross-section view of another configuration of the firstembodiment for gas lasers. If the presence of the transparent substratewith an antireflection multilayer within the gas laser cavity at theside of the gas column is undesired, the device according to theinvention can be placed at the side of the gas column as illustrated inFIG. 14. Direct wafer bonding or anodic bonding is used to perform theadhesion of the gas laser window 70 with the multilayer with theadditional pair of layers 30 supported by a provisional substrate 10.After adhesion between the laser window and the layer 32 with grating40, the substrate 10 is removed by selective wet etching or by means ofa smart cut technique. This allows to avoid detrimental scattering dueto the presence of the corrugation within the laser cavity.

FIG. 15 is represents another structure of the device according to anyembodiment of the invention involving a curved substrate 10 andrepresenting the focusing mirror or coupler of a stable laser resonator.The cross-section view is here shown only for the first embodiment ofthe coupling means. The standard multilayer 20 is similarly coated withthe pair 30 of layers 31 and 32. Depending on the radius of curvature,the grating 40 can either have a constant period or have a groove periodΛ which varies so as to couple the non-collimated incident wavebelonging to a laser resonator mode to a defined mode of layer 32 overthe whole cross-section.

The device according to the present invention comprises a substrate, amultilayer mirror, and coupling means comprizing a corrugation grating.The grating is located in the last layer of the mulilayer (second andthird embodiments) or in (or on) the last high index layer (firstembodiment).

It appears from the above that the invention concerns coupling meansmade of a corrugation grating and a substructure of the device (the lasthigh and low index layers in the first embodiment, the metal ordielectric substrate in the other embodiments) whereby the coupling ofthe incident laser beam of one polarization into a mode of the saidsubstructure results in the decrease, for example by at least 10%, ofthe multilayer reflection coefficient of the said polarization over alarge optical wavelength range. The mentioned figure of 10% is onlyindicative. The requested decrease of the reflection coefficient dependson the laser operation (CW or pulsed regime) and on the gain of theactive medium. Those familiar to the art of lasers will easily state thenecessary reflection coefficients leading to the desired polarizationfiltering.

Multilayer mirrors closing a laser cavity can broadly be of twocategories. The multilayer is called a “mirror” if its function is toreflect close to 100% of the incident energy into the cavity. It iscalled a “coupler” if its function is to partially reflect and partiallytransmit the laser beam. A device according to the invention can be ofone or the other category, as appears clearly from the abovedescription.

The polarization states are said transverse magnetic (TM) if theelectric field is in a plane normal to the grating lines and transverseelectric (TE) if the electric field is parallel to the grating lines.

The above descriptions give only examples of embodiments of the deviceaccording to the invention. A number of other embodiments will be easilydesigned by those familiar to the art in the light of the presentdisclosure.

What is claimed is:
 1. An optical device comprising a substrate, amultilayer mirror, a pair of low and high refractive index layersdisposed on said multilayer mirror, and a diffraction grating in saidhigh refractive index layer, said grating having a period adjusted so asto achieve coupling of one of the polarizations of an incident beam to amode of said high refractive index layer.
 2. A device according to claim1, the thickness and index of the pair of low and high index layersbeing such that the additional optical path resulting from this pair oflayers provides a total round trip additional optical phase equal to aninteger number of essentially 2π at a wavelength of operation λ.
 3. Adevice according to claim 1, the index n_(h) and the thickness t_(h) ofthe high refractive index layer being such that the effective indexn_(e) of the mode of the high refractive index layer to which anundesired polarization is coupled is larger than the index n_(s) of saidsubstrate.
 4. A device as in claim 1, the following condition in thespatial frequency domain being fulfilled: K _(g) =n _(e) k ₀  (2), whereK_(g)=2π/Λ is the grating spatial frequency, k₀=2π/λ is the spatialfrequency of a free space wave in vacuum, and n_(e) is the effectiveindex of a mode of the high index layer.
 5. A device according to claim1, the product of the field radiation coefficient α by an incident beamdiameter w, αw, being larger than
 1. 6. A device according to claim 1,the grating being a rectangular grating of depth d of line/space ratio1:1, d being such that: s=0.5(4d/π), where s is the amplitude of thefirst harmonics in the Fourier series of the grating profile.
 7. Adevice according to claim 1, the grating being a rectangular grating ofdepth d such that: d=(2s)π/4, where s is the amplitude of the firstharmonics in the Fourier series of the grating profile; and t_(eff) isthe known effective thickness of a mode given byt_(eff)=t_(h)+1/(k₀(n_(e) ²−n₁ ²)^(1/2))+1/(k₀(n_(e) ²−1)^(1/2)), where:k₀=2π/λ is the spatial frequency of a free space wave in vacuum, t_(h)is the thickness of the last high index layer, and the index n_(e) andn₁ are, respectively, the effective index of said mode in the high indexlayer for said mode, and the index of the low index layer.
 8. A deviceaccording to claim 1, said multilayer mirror comprising alternate layersof a low index material and a high index material, said pair of low andhigh refractive index layers being made of materials respectivelyidentical to said low index material and high index material of saidmultilayer mirror.
 9. A device according to claim 1, said pair of lowand high refractive index layers being respectively made of SiO₂ andTa₂O₅.
 10. A device as in claim 1, said multilayer mirror being locatedbetween said substrate and said pair of low and high refractive indexlayers.
 11. A device according to claim 1, said substrate being a laseractive material.
 12. A device according to claim 11, said laser activematerial being a Nd:YAG laser material.
 13. A device according to claim1, said substrate being an amorphous material.
 14. A device according toclaim 1, said substrate being a birefringent crystal.
 15. An opticaldevice as in claim 1, said substrate being the window of a laser furthercomprising a gas column, said multilayer mirror being turned toward saidgas column and said high index layer and said grating being turnedtowards said substrate.
 16. A laser mirror comprising a metal substrateor a substrate coated with a metal film, a multilayer mirror, and adiffraction grating, said multilayer mirror being located between saidsubstrate, or said film, and said grating, said grating having a periodadjusted so as to achieve coupling of one of the polarizations of anincident beam to a mode of said multilayer mirror.
 17. A laser mirroraccording to claim 16, further comprising cooling means disposed insidethe substrate.
 18. A mirror as in claim 16, said multilayer mirror andsaid substrate having at least one guided mode of field absorptioncoefficient α_(a), said mode having a field radiation coefficient of αin the presence of said grating which is equal or essentially equal tosaid field absorption coefficient.
 19. A mirror as in claim 16, theperiod Λ of the grating being adjusted so as to satisfy the couplingsynchronism condition Λ=λ₀/n_(e) where λ₀ is the central wavelength ofthe linewidth of a propagating mode of said multilayer mirror, and n_(e)is the effective index of said propagating mode at wave length λ₀ in themultilayer.
 20. An optical device comprising a dielectric substrate, amultilayer mirror, and a diffracting grating, said multilayer mirrorbeing located between said substrate and said grating, said gratinghaving a period adjusted so as to achieve coupling of one of thepolarizations of an incident beam to one of the first order leaky modesof the same polarization of the multilayer and the substrate.
 21. Adevice as in claim 20, said multilayer and said substrate having atleast one leaky mode of field leakage coefficient α₁ and of fieldradiation coefficient α in presence of the grating, α being equal oressentially equal to α₁.
 22. A device as in claim 20, wherein saidgrating has a period Λ=λ₀/n_(e) where λ₀ is the vacuum wavelength of anincident beam and n_(e) is the effective index of a leaky mode.
 23. Anoptical device as in claim 1, said substrate being curved and formingthe focusing mirror of a stable laser resonator.
 24. An optical deviceas in claim 1, the grating comprising grating lines which areazimuthally distributed.
 25. An optical device as in claim 1, thegrating having a spatial frequency spectrum which contains two spatialfrequencies, Kg1 and Kg2, each corresponding to the coupling of oneincident wavelength into the same mode of the grating waveguide formedby the high refractive index layer and the diffraction grating.
 26. Anoptical device as in claim 1, said grating comprising contiguousspatially multiplexed grating areas.
 27. An optical device as in claim1, said grating comprising two grating areas of parallel orientation anddifferent periods.
 28. An optical device as in claim 1, said gratingcomprising two grating areas of orthogonal orientation and differentperiods.
 29. A microchip laser comprising an active solid medium, afirst and a second mirrors, one of said first and second mirrorscomprising a device according to claim
 1. 30. A method of damping afirst polarisation of an incident light comprising a first and a secondpolarisation components, said method comprising directing said beam oflight toward an optical device, comprising a substrate, a multilayermirror, a pair of low and high refractive index layers disposed on saidmultilayer mirror, and a diffraction grating in said high refractiveindex layer, whereby said first polarisation is coupled into a mode ofsaid last index layer and is reflected back from the grating with aphaseshift of π.
 31. A method of damping a first polarisation of anincident light comprising a first and a second polarisation components,said method comprising directing said beam of light toward an opticaldevice according to claim
 1. 32. A method of damping a firstpolarisation of an incident light comprising a first and a secondpolarisation components, said method comprising directing said beam oflight toward an optical device, comprising a metal substrate or asubstrate coated with a metal film, a multilayer mirror, and adiffraction grating, said multilayer mirror being located between saidsubstrate, or said film, and said grating, whereby said firstpolarisation is coupled into a mode of said multilayer and saidsubstrate.
 33. A method as in claim 31, said mode to which said firstpolarisation is coupled being a guided TM mode.
 34. A method of dampinga first polarisation of an incident light comprising a first and asecond polarisation components, said method comprising directing saidbeam of light toward an optical device, comprising a dielectricsubstrate, a multilayer mirror, and a diffraction grating, saidmultilayer mirror being located between said substrate and said grating,whereby said first polarisation is coupled to one leaky mode of samepolarisation of said multilayer and said substrate.
 35. A deviceaccording to claim 2, the index n_(h) and the thickness t_(h) of thehigh refractive index layer being such that the effective index n_(e) ofthe mode of the high refractive index layer to which an undesiredpolarization is coupled is larger than the index n_(s) of saidsubstrate; the following condition in the spatial frequency domain beingfulfilled: K _(g) =n _(e) k ₀  (2), where K_(g)=2π/Λ is the gratingspatial frequency, k₀=2π/λ is the spatial frequency of a free space wavein vacuum, and n_(e) is the effective index of a mode of the high indexlayer; the product of the field radiation coefficient α by an incidentbeam diameter w, αw, being larger than 1; the grating being arectangular grating of depth d of line/space ratio 1:1, d being suchthat: s=0.5(4d/π), where s is the amplitude of the first harmonics inthe Fourier series of the grating profile; the grating being arectangular grating of depth d such that: d=(2s)π/4, where s is theamplitude of the first harmonics in the Fourier series of the gratingprofile; and t_(eff) is the known effective thickness of a mode given byt_(eff)=t_(h)+1/(k₀(n_(e) ²−n₁ ²)^(1/2)) +1/(k₀(n_(e) ²−1)^(1/2)),where: k₀=2π/λ is the spatial frequency of a free space wave in vacuum,t_(h) is the thickness of the last high index layer, and the index n_(e)and n₁ are, respectively, the effective index of said mode in the highindex layer for said mode, and the index of the low index layer; saidmultilayer mirror comprising alternate layers of a low index materialand a high index material, said pair of low and high refractive indexlayers being made of materials respectively identical to said low indexmaterial and high index material of said multilayer mirror; said pair oflow and high refractive index layers being respectively made of SiO₂ andTa₂O₅.
 36. A device as in claim 35, said multilayer mirror being locatedbetween said substrate and said pair of low and high refractive indexlayers.
 37. A device according to claim 36, wherein said substrate beingselected from the group of an amorphous material and a birefringentcrystal.
 38. An optical device as in claim 35, said substrate being thewindow of a laser further comprising a gas column, said multilayermirror being turned toward said gas column and said high index layer andsaid grating being turned towards said substrate.
 39. A mirror as inclaim 17: said multilayer mirror and said substrate having at least oneguided mode of field absorption coefficient α_(a), said mode having afield radiation coefficient of α in the presence of said grating whichis equal or essentially equal to said field absorption coefficient; theperiod Λ of the grating being adjusted so as to satisfy the couplingsynchronism condition Λ=λ₀/n_(e) where λ₀ is the central wavelength ofthe linewidth of a propagating mode of said multilayer mirror, and n_(e)is the effective index of said propagating mode at wave length λ₀ in themultilayer.
 40. A device as in claim 21, wherein: said grating has aperiod Λ=λ₀/n_(e) where λ₀ is the vacuum wavelength of an incident beamand n_(e) is the effective index of a leaky mode; said substrate beingcurved and forming the focusing mirror of a stable laser resonator. 41.An optical device as in claim 40, the grating comprising grating lineswhich are azimuthally distributed; and the grating having a spatialfrequency spectrum which contains two spatial frequencies, Kg1 and Kg2,each corresponding to the coupling of one incident wavelength into thesame mode of the grating waveguide formed by the high refractive indexlayer and the diffraction grating.
 42. An optical device as in claim 41,said grating comprising contiguous spatially multiplexed grating areas.43. An optical device as in claim 42, said grating comprising twograting areas of parallel orientation and different periods.
 44. Anoptical device as in claim 40, said grating comprising two grating areasof orthogonal orientation and different periods.
 45. An optical deviceas in claim 41, said grating comprising two grating areas of orthogonalorientation and different periods.
 46. An optical device as in claim 42,said grating comprising two grating areas of orthogonal orientation anddifferent periods.
 47. A microchip laser comprising an active solidmedium, a first and a second mirrors, one of said first and secondmirrors comprising a device according to claim
 44. 48. A microchip lasercomprising an active solid medium, a first and a second mirrors, one ofsaid first and second mirrors comprising a device according to claim 45.49. A microchip laser comprising an active solid medium, a first and asecond mirrors, one of said first and second mirrors comprising a deviceaccording to claim
 46. 50. A method of damping a first polarisation ofan incident light comprising a first and a second polarisationcomponents, said method comprising directing said beam of light towardan optical device according to claim
 44. 51. A method of damping a firstpolarisation of an incident light comprising a first and a secondpolarisation components, said method comprising directing said beam oflight toward an optical device according to claim
 45. 52. A method ofdamping a first polarisation of an incident light comprising a first anda second polarisation components, said method comprising directing saidbeam of light toward an optical device according to claim 46.